Electro-mechanical transducer



Nov. 19, 1963 E. v. CARLSON ELECTRO-MECHANICAL TRANSDUCER 2 Sheets-Sheet1 Filed May 5, 1960 INVENTOR. am %Mm Nov. 19, 1 963- E. v. CARLSONELECTRO-MECHANICAL TRANSDUCER Filed May 5, 1960 2 Sheets-Sheet 2 UnitedStates Patent 3,111,563 ELEUTRO-MECHANHCAL TRANSDUCER Elmer V. Carlson,Prospect Heights, Ill, assignor to lindustrial Research Products, Inc,Franklin Park, 111., a

corporation of Delaware Filed May 5, 1960, Ser. No. 27,006 18 Claims.(U. 179-114) This invention relates to an electro-mechanical transducerof the type employing a magnetic armature having a portion vibratable inan air gap in a steady magnetic circuit. It also relates to anelectro-acoustic transducer which employs the electro-mechanicaltransducer in conjunction with a diaphragm or stylus. In this type oftransducer, a permanent magnet provides a steady flux at an air orworking gap. This flux is referred to herein as the steady flux, and theflux conductors and magnet or magnets constitute the steady fluxcircuit. There is a coil around the armature which, when the transduceris used as a receiver, is energized so as to induce a flow of flux alongthe armature, and when the transducer is used as a microphone, fluxflowing along the armature induces a potential in the coil. Flux movingalong the armature as a result of a signal current in the coil, or as aresult of vibration of the armature, is called herein the signal flux,and the flux conductors forming this flux circuit are called the signalflux circuit.

The principal object of this invention is to enclose the signal fluxcircuit substantially entirely within an encompassing steady fluxcircuit. A feature of this invention is the positioning of aflux-conductive case in the steady flux circuit so as to besubstantially the sole magnetic conductor in part of the steady fluxcircuit. Contrasting this object and feature with existing transducerdesign, in the latter, the electro-mechanical transducer is a completeassembly within a case. The case, whether magnetic or nonmagnetic, isspaced from a complete electro-mechanical transducer therein. That is tosay, the steady fiux circuit of present transducers utilizes fiuxconductors which perform the flux-conductive function and little more.While the conductors of the steady flux circuit may be related to theconductors of the signal fiux circuit, they are more often than notarranged to separate the flux fields from the two circuits rather thanto combine them. Separation of the two circuits is desirable. Thegeneral object of this invention may also be expressed in this Waytoenclose the signal flux circuit completely within the steady fluxcircuit, or again, to enclose the flux field of the signal flux circuitentirely within any fields created by the steady flux circuit. As willappear, applicants case at all points is at substantially a commonmagnetic potential with the result that flux fields originating outsideof the case will affect only the steady flux circuit and will notpenetrate to the signal flux circuit. Conversely, the signal fluxcircuit and fields generated by it will be confined within the case.

The second object of this invention is to engage the poles of one ormore magnets with the inside wall of a flux-conductive case in such amanner as to retain the desirable common magnetic potential throughoutthe case.

A feature of this invention is a magnet stack consisting I of twoelongated, spaced magnets, transversely magnetized with their outerpoles of opposite polarity and in physical engagement with oppositeinside walls of the case.

Another object of this invention is to center substantially an armatureWithin the flux-conductive case so that the armature is at very nearlythe same steady magnetic potential as the case.

Another object of this invention is to utilize a freely vibratableE-shaped or U-shaped armature. A feature of the invention is themounting of the ends of the outer arms of an E-shaped armature fixedlyin the space be- 3,1 l 1,5 63 Patented Nov. 19, 1963 tween the twomagnets so that the entire armature may vibrate freely and not merelythe end of the center arm which is disposed in an air or working gapbetween the magnets. This is to be contrasted with structures holdingthe E-shaped armature along its base which necessarily involvesestablishing a fixed relationship between such a holding means and themagnet.

An ancillary object is to limit transverse movement of the E- orU-shaped armature to a point well within the range wherein movement ofthe armature Will exceed the elastic limit of its material. If thearmature should move a greater distance, it may take a permanent setwith permanent injury to the transducer. Applicant employs one of twomeans to accomplish this object. One means holds the armature at a nodeof vibration. This leaves the armature free to vibrate, but ifthetransducer is struck as by hitting a floor and the shock issufiicient so that the mass of the armature would deflect it so as toexceed its elastic limit, usually near the fixed gap, the holding meansat the node of vibration will provide .the necessary support. Analternative means is the positioning of stops on either side of thearmature and supported by the case at a distance such that deflection ofthe armature is held within safe limits, although permitting freevibration in normal operation.

These and such other objects of the invention as may hereinafter appearare attained in the embodiment of the invention hereinafter describedand shown in the drawings, wherein:

FIGURE 1 is a perspective view, partly cut away and partly exploded, ofapplicants electro-acoustic transducer with the coil shownschematically;

FIGURE 2 is a top plan View, partly cut away;

FIGURE 3 is a view taken on the line 33 of FIG- URE 2;

FiGURE 4 is an elevation of that side showing the coil and lid terminalposts;

FIGURE 5 is a view taken on the line 5-5 of FIG- URE 2; and,

FIGURE 6 is a fragmentary view of the end of the coil assembly engagingthe central arm of the E-shaped armature at its node of vibration.

Continuing to refer to the drawings, the transducer motor consists of aninternal assembly and a flux-conductive case. The internal assemblyconsists of a magnetic stack and armature and coil. The magnetic stackcomprises a magnet 10, flux-conductive pole piece '12, a spacer of lowflux conductivity 14, an E-shaped armature 16 having side arms 18 and 20and a central vibratable arm 22, a second spacer of low fluxconductivity 24, a second flux-conductive pole piece 26, and .asecondmagnet 28. The magnets are transversely magnetized as indicated. Thepole pieces 12 and 26 are coextensive with the magnets ill and 28-. Forsimplicity in construction, the two space-rs 14 and 24 may be made froma single U shaped member of low flux conductivity which is slipped overthe end of the arm 29 of the anmature 16. Around the arm 18 is a secondpair of spacers 30 of low flux conductivity. A coil 32 surrounds thecenter arm 22. of the E-shaped armature and is spaced therefrom. Betweenthe working end of the central arm 22 and the pole pieces 12 and 26,there are no spacers, but an air gap in which the anmature arm 22 isfree to vibrate. This is called the working gap. The numerals witharrowhead lead lines 122. and 124 identify the fixed magnetic gaps, andthe numeral 126 identifies the Working gap.

The foregoing describes the complete motor assembly, which istemporarily held in assembled relationship by any suitable means such asriveB or adhesives. This motor assembly however, would be highlyinefiicient because there is no good flux conductor connecting the northpole of magnet 10 to the south pole of magnet 28. This conductor issupplied by the case which consists of a bottom receptacle or cup 34made of material of high flux-conductivity with a peripheral shoulder 36around the inside of its side walls. Seated against this shoulder is aclosure or cup 38, also made of flux-conductive material. Referring toFIGURE 5, the peripheral wall 40 of the closure 38 is in good magneticconnection with the upper part of the side walls of the bottomreceptacle 34 to complete a flux-conductive closed path through the twocups 34 and 38' which combinedly form what may be called aflux-conductive envelope.

With the parts thus far described, one has a highly eflicientelectro-acoustic motor. The steady magnetic flux from the magnets and 28is provided with an excellent flux-conductive path through the closure38 and the bottom cup 34a fiux path so uniform that the entire case issubstantially at a common potential. Whatever flux fields are developedwithin the case by the movements of the armature or a current flowingthrough the coil, will be contained by this flux-conductive case, andsimilarly any stray fields impinging upon the case will be absorbedthere in the steady flux circuit and not penetr-ate to the armature andcoil. As is well understood, where the armature moves as a result ofmechanical action, flux will flow in one or the other direction down thecentral arm 22 of the armature 16 and around the outside arms 20 and 18through the fixed gap of the magnets to complete the circuit through oneor the other of the pole pieces. The flux lines are generally indicatedby the dash lines 153. Where the source of energy is current in thecoil, reversal of direction of current in the coil causes reversal ofdirection of the flux along the arm 22 and the circuit is completed asbefore. Referring to FIGURES 1 and 5, the closure 38 is centrallyapertured and a diaphragm 40 is mounted on the outside of the closure38. Centrally of the diaphragm 40 is anchored a link 42 which is alsoanchored on the central arm 22 of the armature -16. Mounted over thediaphragm is a lid 44 which is preferably of flux-conductive materialwhich has a central opening 46, shoulder 48, and an insert 50 having asmall central opening 52. This lid 44 is pressed against the bottomreceptacle 34 adjacent the closure 38. The steady magnetic potential ofthe lid 44- is close to the common potentiol of the direct flux circuitof the bottom receptacle 34 and the closure 38.

In speaking of the case in this invention, the lid 44 may in fact be thecase, but this is because the diaphragm 40 is located outside of theclosure 38. The diaphragm can be positioned inside the closure underwhich circumstances the lid 44 would not be used. Similarly, if a styluswere connected to the armature, it would occupy a position such as thatof the drive link 42.

The signal flux circuit, therefore, consists of the armature 16 and thepole pieces 12 and 26. The steady flux circuit consists of the magneticstack and the magnetic case. The two circuits intercept each other atthe fixed gaps 122 and 124- and the working gap 126. The entire fluxcircuit is within the steady flux circuit whose major conductor, thecase, both shields the signal flux circuit from fields external to thecase and restricts signal fiux fields to the case.

When the diaphragm or stylus is added to the electromechanicaltransducer described, one has an electroacoustic transducer.

Describing other elements of the invention, the numeral 54, referring toFIGURE 3, identifies an inertance tube which opens into the back ormotor cavity 56 of the transducer, and also into the shallow frontcavity 58, see also FIGURE 2.

The coil of wire 32 extending approximately to the dotted lines 60 inFIGURE 4 is either Wound on a bobbin or encased in at least a partialenclosing shroud having the general configuration of the dotted lines62, and having the recesses 64 and 66 provided to provide an upper andlower limit of movement of the arms 18 and 20 of the armature in theevent of shock. Lead wires 73 and 75 electrically extending the ends ofthe winding, may also have insulating shrouds 72, 74, 6 8 and 70 servingto space the coil within the housing and to insulate the lead wires 73and 75 from the housings. Alternatively, the excess material 62 exteriorto the winding 60 may be eliminated and the coil positioned by placingadhesive cement between the coil 32, the housing 34 and/or the housing38, the coil in this construction not being in physical contact with anyportion of the armature '16.

When one of these microphones or receivers is dropped on a hard surfacefrom a distance of several feet or more, the vibrating components suchas the armature may be subjected to an acceleration of several thousandtimes that of gravity. Even when one is mounted in a complete device,such as a hearing aid, in the normal manner with compliant supports, ithas been found that the acceloration may exceed 2000 times gravity whenthe complete device is dropped a distance of five feet. Accelerations ofthis magnitude in the direction of motion of the armature make thepresent type transducer inoperative or seriously impair theirperformance.

To provide performance characteristics which are independent oftemperature and ageing effects, it is desirable that the armature 16 beso supported that it can expand and contract without constraint and thatany changes in magnet and pole piece position should not infiuence therelative position of the armature center arm 22 with respect to theoperating air gap. The vibration amplitude of the armature has a maximumat the working gap end of the center arm 22 at 126. The amplitudediminishes gradually and is very nearly zero where the center arm 22 isattached to the rear transverse arm 16, along line 76, see FIGURE 1.This amplitude minimum or node may be slightly within transverse member16; that is, slightly back of the plane of the rear end of the coil.Beyond this node, the displacement is normally opposite in direction andincreases towards the corners of the E where the transverse member 16joins the side members 20 and then diminishes rapidly to zero at thepoint of attachment of the members 20 to the spacers 14. This node is avirtual fulcrum and at this point it is possible to provide a supportmember which is quite rigid in a direction normal or vertical to thearmature without sensibly affecting the vibration of the armatureproviding the support member does not prevent the armature from rockingabout this fulcrum region. The supsport member 83 in FIGURE 3 performsthis function. It may be spot welded or otherwise attached to thetransverse armature member 16 near the dotted line near where the centermember 22 joins it, or at approximately the dotted line shown inFIGURE 1. The exact location of the node will depend on the shape of theparticular armature used, but in any event, the preferred arrangement isone in which the support is placed at or near a node but in a way thatpermits substantially normal vibratory motion of the entire armature. Inthe embodiment shown in FIGURE 3, the member 83 is cemented to the rearend of the coil while removable positioning members are placed in theopening in the coil to center the arm 22 in the coil. When this cementis rigid, the positionmg members are removed and the coil and armature,together with the pole pieces, magnets and the rest of the motorsub-assembly are centered within the bottom receptacle or cup 34. Justprior to insertion, a drop of cement 85 is added to the under and uppersides of the coil so that the shock member 83 and the coil 32 arecemented to the cup 34. The cement on the upper part of the coil 32serves to hold it in position with respect to cup 38.

Further shock protection is provided by the notches 64 and 66. These areso dimensioned and positioned with respect to the arms 18 and 20 thatthey do not prevent motion between these arms and the bobbin wall innormal operation. If, however, a high stress is applied to the armatureby sudden deceleration or acceleration, the sides of the slots serve asbumpers or stops to prevent excess motion. For moderate shockrequirements, the shock member 83 ,is not required, if the size of theopening or slots in the coil are so dimensioned that the armature isstopped before it exceeds its elastic limits. In the. structure shownwhich is inch square, it. is desirable to limit the displacement of anypart of the armature to approximately .005 inch.

An alternative means of supporting the armature 16 at its node ofvibration is shown in FIGURE 6 where the numeral. '71 identifies aportion of a metallic or plastic end wall of the coil, corresponding. to62 in FIGURE 3. A flattened diamond-shaped opening is dimensioned toengage the central arm 22 close to the line 76 of FIGURE 1. Movement ofthe wall 71 is constrained by the inside wall of the case, or by beingaffixed to the coil, which is otherwise restrained, as by the cement 85.

Where the transducer is used as a microphone, ideally the magnets are ofidentically the same length because if the geometry and relationship ofall of the parts is perfect, there will be no flow of flux along thearmature. Where the transducer is used as a receiver, the generalpractice of hearing aid manufacturers, is to operate the coil at aselected direct current level with the result that with exactly equalmagnets, there will be a continual flow of flux in one direction alongthe armature. The preferred method of establishing a bucking orcountenfiux so as to balance the steady flux in the armature in thisdesign is to make one magnet longer or stronger than the other and toclamp the armature nearer one pole piece than the other by havingthinner spacers between one pole piece and the clamped armature ends soas to oppose this flux generated by a continuous current through thecoil and exactly neutralize it. In this case, spacers 14 and 30 arereplaced by two thin spacers onv one side and two normal spacers on theother side of the armature.

Applicant has shown an E-shaped armature. A U- shaped armature may beused, in which case the total area of the armature in the fixed gap willapproximately equal the total area of the armature in theworking gap.Where the E-shaped armature is used, the combined areas of the two endsof the arms.18 and 29 in the fixed gaps equals the total area of the arm20 in the working gap. For ideal operation, the total area of the bottomreceptacle 34 and the closure member 38 should be approximately the samein order to establish a uniform steady flux potential in the case as aWhole.

The electro-acoustic transducer described has a very low sensitivity toexternal fields (approximately 1% of conventional types), when used as amicrophone, and generates very little external field when used as areceiver. For a given separation, this permits up to 40 decibels moregain Without instability or permits mounting more closely together(within a few thousandths of an inch) with a conventional amplifier.This is done without any auxiliary magnetic shield that requires veryprecious space. Even a moderate acoustical pressure amplification of 100times (4-0 decibels) in a typical hearing aid necessitates an electricalpower amplification of 100 million times (80 decibels) or more. As theaid is made more compact, the separation between the microphone andreceiver is necessarily reduced if they are mounted in the same housing.This is normally the practice in back-of-the-ear aids. Since themagnetic coupling varies roughly inversely with the cube of theseparation, the magnetic coupling problem becomes acute. Magneticshields external to the magnetic circuits of the transducer and externalto the complete transducer reduce the coupling but require a prohibitiveamount of space, and also increase the magnet leakage field, therebyimpairing the performance. Even though positioned to minimize coupling,the relative positioning for minimum coupling becomes too critical andcannot be preserved in production or in use. The design shown hereinreduces the electrical power induced in a microphone by a receiver (bothof this construction) by a factor of nearly 10,000 (i.e., an inducedvoltage ratio of nearly compared to the commonly used types. This makesit possible to mount the two transducers very close together without.excessive magnetic coupling in some applications. It also results in amore stable performance of the aid since the relative microphone andreceiver orientations are not critical.

An important advantage of the use of the case as substantially the soleiiux conductor of a major portion of the steady flux circuit is that oneeliminates at least two thick.- nesses of flux conductors in theassembly. This substantially decreases the volume of the case andimportantly, narrows the case along the. dimension which is at rightangles to the plane of the diaphragm.

The lid or top cover 44, FIGURES 1 and 3, when made of magneticmaterial, performs two functions: That of an additional magnetic shieldand that of forming the major wall of the front acoustic cavity 58. 'Dueto its good contact with the upper cup 38 and the lower cup 34, thecover 44 is substantially at the common potential.

The transducer shown is .375 inch square and .185 inch thick, thesebeing the over-all outside dimensions. The parts are very small. Thearrangement of the conductors comprising the two flux circuits, magnetsand coil, is important, therefore, not only from the standpoint offunction, but also because'of their providing great compactness.

Having thus described my invention, what I claim is:

1. An electro-mechanical. transducer comprising a steady flux circuitwhich includes a first and a second permanent magnet spaced from oneanother with their opposite. polesconfronting across aworking gap, and aflux conductor connecting the remote poles of the magnets to completethe steady flux circuit to the working gap; and a signal flux circuitwhich includes an armature having one end fixed with respect to themagnets and the other end vibratable in the working gap, and a coilaround a portion of the armature; the flux conductor of said steady fiuxcircuit-comprising a substantially continuous, fluxconductive envelope.

2. An electro-acoustic transducer comprising a steady ii'ux circuitwhich includes an enclosing flux-conductive envelope, two permanentmagnet means having opposite poles contacting the interior wall of theenvelope at spaced points to fiux-conductively connect said oppositepoles, said magnets being spaced from each other with their otheropposite poles confronting one another across a working gap in saidsteady flux circuit within the envelope;.a signal flux circuit whichincludes an armature having one end mounted in fixed relation to saidmagnets and having a vibratable portion in the working gap of the steadyflux circuit, and a coil around the. armature; a port through one'wallof the envelope, a drive link mounted on the vibratable portion of thearmature and extending through the port, and acoustic means on the free.end of the link.

3. The electro-acoustic transducer of claim 2 wherein the acoustic meansis a diaphragm.

4. The electro-acoustic transducer of claim 2 Wherein the acoustic meansis a diaphragm mounted on the outside wall of the envelope and amagnetic cover containing a sound port is mounted over the diaphragm influx-conductive relationship to the envelope.

5. An electro-mechanical transducer comprising a pair of spacedpermanent magnets with their poles in series relationship, the spacebetween the magnets including an air gap, an armature and coiltherearound positioned adjacent the magnets, one end of the armaturebeing fixed between spacers of low flux conductivity in the spacebetween the magnets and the other end being vibratable in the air gap,and a flux-conductive case engaging the outer opposite poles of the twomagnets to form around the entire assembly a substantially closedcircuit for the steady flux from the permanent magnets.

6. The electro-mechanical transducer of claim wherein the magnets areelongated and magnetized transverseiv to their length and the armaturehas two parallel arms joined at one end of each, the free end of one armbeing mounted in fixed relationship between the magnets and the free endof the other arm being vibratable in the working gap.

7. The electro-mechanical transducer of claim 6- together with meansfixed with respect to the case for holding the armature.

8. The electro-mechanical transducer of claim 6 together with meansfixed with respect to the case for stopping lateral displacement of thearmature beyond its elastic limit.

9. An electro-mechanical transducer comprising a magnetic bottom cup,having a flat bottom portion, a permanent magnet having one polecontacting said flat bottom portion, a spacer mounted on the other poleof said magnet, a U-shaped armature having the end of one arm engagingsaid spacer, a second spacer mounted on the same arm of the armature, asecond like permanent magnet mounted on the spacer, the end of thesecond arm of the armature being vibratable in air between the magnets,a coil around one arm of the armature, and a magnetic top cup having afiat top portion, the inside wall of said fiat top portion contactingthe pole of the second magnet opposite to the pole of the first magnet.

10. An electro-mechanical transducer comprising a magnetic bottom cuphaving an elongated flat bottom surface, an elongated permanent magnettransversely magnetized having one pole contacting said fiat bottomportion, a spacer mounted on each end of the opposite pole of saidmagnet, an E-shaped armature having the end of each outer arm mounted onone spacer, a spacer mounted on the end of each outer arm, a liketransversely magnetized magnet mounted on the second spacer, the end ofcentral arm being positioned and vibratable between the magnets, a coilaround a portion of the armature, and a magnetic top cup having a flattop portion contacting that pole of the second magnet opposite to thepole of the first magnet.

11. The electromechanical transducer of claim 10 together with meansholding the E-shaped armature in fixed relationship to the cups near theline of joinder of its central arm to the base of the E.

12. The electro-mechanical transducer of claim 10 together with aflexible bracket having one end fastened to one cup and the other endflexible in the plane of the armature fastened to the armature near theline of joinder of its central arm to the base of the E.

13. The electro-mechanical transducer of claim 10 together with a thinend Wall mounted on the coil, said thin end wall having an openingtherethrough of a configuration and size such as to engage the oppositeedges of the central arm of the armature near the node of vibration ofthe armature. 1

14. The electro-mechanical transducer of claim 10 wherein a pole pieceis positioned between each pole face and the adjacent spacer. l

15. An electro-mechanical transducer comprising a steady flux circuitwhich includes two spaced, permanent magnets with opposite poles facingeach other on either side of a working gap and a fixed gap, and amagnetic case connecting the second poles of the magnets respectively toeach other; and a signal flux circuit which includes a pole piece oneach magnet gap face, an armature having one portion fixedly mountedbetween spacers of low flux conductivity in the fixed gap and having itsother end vibratable in the working gap, and a coil around a portion ofthe armature; said case substantially enclosing all of the elements ofthe signal flux circuit.

'16. An electro-mechanical transducer comprising two permanent magnetsspaced from each other to form a working gap with the opposite poles ofthe magnets facing each other, an armature mounted in fixed relationshipto the magnets and having a vibratable portion in the working gap, acoil around the armature, and flux-conductive members enclosing theforegoing elements, said flux-conductive members being influx-conductive engagement with the outer poles of both magnets so as toform a part of the steady flux circuit of the transducer andsubstantially completely enclose the working mechanism Within thatcircuit. 17. An electro-acoustic transducer comprising two permanentmagnets spaced from each other to form a working gap with the oppositepoles of the magnets facing each other, an armature mounted in fixedrelationship to the magnets and having a vibratable portion in theWorking gap, a coil around the armature, a flux-conductive cup in whichthe foregoing assembly is seated with the outer pole of one magnet influx-conductive relationship With the cup, a flux-conductive lidcovering the cup and engaging the outer pole of the other magnet so thatthe cup and the lid form an enclosing portion of the steady fiux circuitfrom the magnets, an opening through said lid, a drive pin mounted on avibratable portion of the armature and extending through said opening, adiaphragm mounted externally of the lid and connected to the outer endof the drive pin, and a second lid mounted on the first lid over thediaphragm, there being a sound opening through the second lid.

18. The electro-acoustic transducer of claim 17 wherein the second lidis made of flux-conductive material.

References Cited in the file of this patent UNITED STATES PATENTS2,454,425 Bauer Nov. 23, 1948 2,912,523 Knowles et al. Nov. 10, 19592,994,016 Tibbetts et al July 25, 1961 3,076,062 Fener Ian. 29, 1963FOREIGN PATENTS 1,044,169 Germany Nov; 20, 1958

2. AN ELECTRO-ACOUSTIC TRANSDUCER COMPRISING A STEADY FLUX CIRCUIT WHICHINCLUDES AN ENCLOSING FLUX-CONDUCTIVE ENVELOPE, TWO PERMANENT MAGNETMEANS HAVING OPPOSITE POLES CONTACTING THE INTERIOR WALL OF THE ENVELOPEAT SPACED POINTS TO FLUX-CONDUCTIVELY CONNECT SAID OPPOSITE POLES, SAIDMAGNETS BEING SPACED FROM EACH OTHER WITH THEIR OTHER OPPOSITE POLESCONFRONTING ONE ANOTHER ACROSS A WORKING GAP IN SAID STEADY FLUX CIRCUITWITHIN THE ENVELOPE; A SIGNAL FLUX CIRCUIT WHICH INCLUDES AN ARMATURE